Familial hypophosphatemic rickets is an x-linked disorder of phosphate homeostasis. Affected patients suffer a variable degree of limb deformity and disproportionate short stature. The pathophysiology of XLH is dominated by, but not limited to, renal phosphate wasting. This disorder has long been attributed to a primary defect in renal phosphate transport. This now appears unlikely for three reasons: neither of the two, recently cloned renal phosphate tranporters is n the x-chromosome, (1,2); current evidence suggests that the renal phosphate loss in HYP, the Murine equivalent of XLH, is due to the action of a humoral factor or factors on normal kidney and not due to an intrinsic renal defect; the XLH defect involves more than an abonormality of phosphate transport since these patients have reduced renal calcitriol generation in response to PTH and decreased phophate levels.